In prior amplifier technology, it has been difficult to design a high-efficiency amplifier which also exhibits low distortion. In the present specification, an amplifier is taken to include its associated power supply, and a low-distortion amplifier is defined as one whose signal transfer function closely approximates the desired or required transfer function. The desired transfer function for an audio amplifier is linear at all audio frequencies, while in a control system, for example, the desired transfer function may be nonlinear. Both types of transfer function can be realized with high accuracy, here referred to as low distortion, by the implementation of the present invention. High efficiency is a desirable attribute because not only is less power required by the amplifier for a given output, but heat-sink requirements to eliminate the wasted power from the amplifier are reduced. In addition, output devices with a reduced power rating may be employed in the amplifier, or, if there are several output devices, fewer may be needed for a given amplifier power rating. Reducing the heat-sink requirements, reducing the necessary output-device ratings, and reducing the number of output devices, all lower the final cost of an amplifier and decrease its size and weight. The cost, size and weight may be further reduced by the elimination of any mains- or audio-frequency transformers carrying substantial power; although, if the mains-frequency transformer is omitted, for many applications the design must still provide mains isolation. An efficient amplifier may also be designed to run at lower temperatures, and as the output devices then suffer less stress, reliability of operation is increased.
Low distortion is also an important attribute for amplifiers. While high-efficiency amplifiers utilizing switching techniques, commonly referred to as Class-D amplifiers, have been known for some time (see, for example: H. R. Camenzind, "Modulated pulse audio power amplifiers for integrated circuits", IEEE Trans. Audio Electroacoust., vol. AU-14, pp. 136-140, Sept. 1966; J. A. Dutra, "Digital amplifiers for analog power", IEEE Trans. Consumer Elec., vol. CE-24, pp. 308-316, Aug. 1978), they have failed to become popular because their distortion is high; low distortion is an essential requirement for high-fidelity audio amplifiers. Another problem with Class-D amplifiers which has limited their acceptance is radiation of high-frequency interference or noise from the amplifier output.
Jensen and Kibakin (references: J. L. Jensen, "Amplifier having series regulated voltage supply", U.S. Pat. No. 3,426,290, Feb. 4, 1969; V. M. Kibakin, "An amplifier with a tracking power supply", Telecommun. Radio Eng., pt. 2, vol. 30, pp. 127-128, Mar. 1975) have described high-efficiency amplifiers which have no switching signals at their output and therefore do not radiate high frequency interference. However, these amplifiers have several defects. They do not exhibit low distortion, their output-stage circuitry is not isolated from the power-supply source, they require audio-frequency transformers one of which carries the total amplifier output and which may need mains-quality insulation, and they do not have high efficiency if the output devices are biased with appreciable current (for Class-A biasing the quiescent current is half the maximum peak current).